1. Field of the Invention
This invention relates to a photographic taking lens driving means which electromagnetically drives a photographic taking lens unit so as to bring a subject into focus or the like.
2. Description of the Prior Art
With the spread of the miniaturization of cameras in size and of collapsible mount type cameras which are so designed as to stow the taking lens barrel into the camera body, it has become difficult to drive various driving mechanisms such as a shutter mechanism, a taking lens driving mechanism, a film winding mechanism and the like by means of a single driving power source such as a built-in motor of a camera body, because of the space for them to occupy. In other words, the reason thereof is that such a driving power source cannot help being bulky if an adequate power should be generated thereby, and the power transmission mechanism must be complicated, so that those mechanisms cannot be stowed in such a small space. Particularly, in a collapsible mount type camera of which the taking lens barrel is retractable, the power transmission mechanism to the taking lens driving mechanism is still more complicated, so that a large space must be taken up for those mechanisms.
Accordingly, the latest models of cameras have a tendency to use electromagnetic driving devices each to every mechanism so as to miniaturize the driving power source thereof and to simplify or disuse the power transmission mechanism thereof.
The similar tendency may be said of the case that a lens unit is driven for focusing or the like. There have heretofore been a system using a motor to serve as a driving power source and another system using an electromagnetic force as a driving power source. In the former system, there have been two sub-systems, i.e., one sub-system in which, as seen in lens-shutter type cameras, a taking lens is motivated to the infinite focal point by a spring at the time when a picture was taken and a film was wound up to the next frame and the taking lens is pulled forth by a motor to a position given by a distance measurement signal at the time of taking picture, and the other sub-system in which, as seen in the automatic focusing devices of single lens reflex type cameras, a taking lens is driven according to the variations of a motor revolution rate and the direction thereof demanded by a distance measurement signal. Both of the systems must be supplemented by a reduction gear, a device for converting a rotation into a rectilinear motion and the like. It has therefore become difficult to incorporate them into the latest models of cameras which are being miniaturized and lightened.
Resultantly, the electromagnetic driving system that is the latter system has been adopted. Such an electromagnetic driving means for driving a taking lens to bring into focus has been designed, on the premise that a loss possibly caused on a magnetic circuit may be reduced by narrowing the gap between the fixed permanent magnets which are arranged opposite to each other, so that the taking lens unit may be driven by a movable coil member arranged inside the gap. Therefore, this means cannot select any but the two-step control in the two directions of currents applied, i.e., one direction from the infinity to the closest range and the other direction from the closest range to the infinity, so that the taking lens may not be moved multistepwise. And yet, it is somehow possible to keep the taking lens unit at the mid point of the driving range thereof by utilizing the balance of a spring or by making use of a mechanical means so that the taking lens may be driven back and forth from the mid point. In this sense, it can not always say that a three-step control system may not be taken by using a piece of movable coil member. A spring tension not affected by an attitude variation is required and an electromagnetic force must be stronger than the spring tension, in order to keep the taking lens at the mid point by utilizing a spring balance. In order to keep it at the mid point mechanically, it is required to provide a suplementary device for releasing the holding member of the lens immediately before the lens driving. Anyway it was very difficult to materialize such a mechanism as mentioned above.